better test for error case : check response_body structure

tracim/tests/functional/test_session.py

             params=params,
             status=500,
         )
+        assert isinstance(res.json, dict)
+        assert 'code' in res.json.keys()
+        assert 'message' in res.json.keys()
+        assert 'details' in res.json.keys()
 
 
 class TestLoginEndpoint(FunctionalTest):
             status=400,
             params=params,
         )
+        assert isinstance(res.json, dict)
+        assert 'code' in res.json.keys()
+        assert 'message' in res.json.keys()
+        assert 'details' in res.json.keys()
 
     def test_api__try_login_enpoint__err_400__unregistered_user(self):
         params = {
             status=400,
             params=params,
         )
+        assert isinstance(res.json, dict)
+        assert 'code' in res.json.keys()
+        assert 'message' in res.json.keys()
+        assert 'details' in res.json.keys()
 
     def test_api__try_login_enpoint__err_400__no_json_body(self):
         res = self.testapp.post_json('/api/v2/sessions/login', status=400)
+        assert isinstance(res.json, dict)
+        assert 'code' in res.json.keys()
+        assert 'message' in res.json.keys()
+        assert 'details' in res.json.keys()
 
 
 class TestWhoamiEndpoint(FunctionalTest):
             )
         )
         res = self.testapp.get('/api/v2/sessions/whoami', status=401)
+        assert isinstance(res.json, dict)
+        assert 'code' in res.json.keys()
+        assert 'message' in res.json.keys()
+        assert 'details' in res.json.keys()